Hi @kirfed,
I have also faced a similar issue for my website (nam thủy corp) 1 year ago. Request you to please check your server configuration to make sure that there is no restriction related to maximum file size upload.
Also as @tneville suggests, please check is there any configuration for your WordPress theme that not allow you to upload high-resolution images.
Hope, it will help you to fix the issue.
Many Thanks,
Nam Thuy
